Instrument cables (as well as measuring leads and the like) are usually non-symmetrical lines having a coaxial structure. Viewed from the outside to the inside, a typical instrument cable includes an outer jacket made of polyvinyl chloride (PVC), a copper helical shield or a copper braiding, a conductive plastic layer made of conductive PVC or polyethylene (PE), a dielectric (conductor insulation) made of solid or foamed polyolefins (PE or PP), and a copper inner conductor. For the electric quality of such a cable, the values for the conductor resistance and the capacitance (conductor/shield) are relevant. Due to the use as a connecting line between high-impedance devices (inductive sensors and high-impedance amplifiers), a microphonic effect or “microphony” occurs as an interfering component. This term is understood to mean noises which are audible in the form of crackling and sizzling upon movement of the cable. Microphony may likewise be indicated in values, with a higher value representing a poorer interference performance and a lower value representing a better interference performance.
In order to curb microphony, in the conventional production process a conductive layer serving as a shield is applied inside the cable over the dielectric directly surrounding the inner conductor, as is shown in EP 0 260 373 A2, for example. The conductivity of this shield has a decisive influence on microphony: the higher the conductivity, the lower the microphonic effect. The conductive layer is usually formed of electrically conductive PE or PVC. PE has a conductivity that is 100 times higher than that of PVC, so that under this aspect PE is basically preferable to PVC.
What presents a problem, however, is the bonding of the conductive layer to the dielectric surrounding the inner conductor. When a dielectric without foaming is used, for example, further processing is necessary to separate the conductive layer from the dielectric, such separation generally leads to the dielectric being torn off. In the case of a foamed dielectric, as known from U.S. Pat. No. 5,523,528, for example, it is impossible to separate the conductive layer from the dielectric. Only the use of a separating agent (e.g. graphite) that is applied between the two components, such as proposed in U.S. Pat. No. 4,678,865, can provide a remedy in this case. But, on the other hand, separating agents generally have the disadvantage that they considerably contribute to a deterioration of the microphony behaviour of a cable.
In conventional cable manufacturing, attempts have therefore been made to counter this problem by using a conductive layer made of PVC, which however does not bond to the dielectric made of polyolefins. The poorer conductivity resulting therefrom leads to an increase in microphony.
It is therefore an object of the present invention to provide a cable having an improved microphony performance while eliminating the drawbacks described above.